1. Field of the Invention
The present invention relates generally to a piezoelectric film switch, and more particularly, to a non-contact piezoelectric snap action switch for use in a keyboard, keypad and the like.
2. Description of the Prior Art
Snap action switches are generally well-known. A conventional snap action switch conducts upon mechanical contact between a metal snap action plate (or snap disc) 10 and metal pins (or conductors on a printed circuit board) 12 as shown in FIG. 1. In such a snap action switch, switch closure is recognized by detecting an electrical short between pin (1) or pin (2) and pin (3). Contact occurs when a sufficient force F is applied to the push-button 14 to cause the metal snap action plate 10 to invert within housing 16 so as to contact pin (3). Pins (1)-(3) are usually connected to other circuitry so that a signal indicative of switch closure can be passed along for further processing.
However, the circuit of FIG. 1 has several disadvantages. Namely, mechanical contact between the metal snap action plate 10 and the pin 3 is unreliable because of the collection of moisture and dirt between the contact points. In addition, further processing circuitry is often required to correct for the effects of "contact bounce". To avoid such problems with mechanical snap action switches, non-contact switches have been proposed.
As described by Ben H. Carlisle in "Piezoelectric Plastics Promise New Sensors", Machine Design (Oct. 23, 1986), pp. 105-110, piezoelectric films have been found to be particularly suited for use in non-contact switches. Piezoelectric film is a flexible, lightweight, tough plastic film which is typically formed of a polarized homopolymer of vinylidene fluoride (e.g., polyvinylidene fluoride or "PVDF"). Piezoelectric film is adaptable to diverse applications because of its piezoelectric nature, whereby a voltage is produced upon mechanical deformation of the piezoelectric element. KYNAR.RTM. piezoelectric film, available from ATOCHEM North America, Inc. (ATOCHEM), is one example of a suitable piezoelectric film.
It is well-known that when a working voltage is applied to the electrodes of a piezoelectric film it will elongate or contract, depending on the polarity of the applied voltage. If the piezoelectric film is exposed to an alternating voltage, it will elongate and contract as the polarity changes. Moreover, it is also well-known that when a external force is applied to the film (e.g., compressive or tensile strain) the film develops a proportionate open circuit voltage. Exposure to a reciprocating force results in a corresponding alternating electrical voltage signal across the piezoelectric film.
For most applications, a piezoelectric film switch can be made by laminating a piezoelectric film to one surface of a thin, flat spring. When a stress is applied to the film, such as by deflecting the spring, a voltage pulse is provided. A "snap action" piezoelectric switch of this type is described by Fanshawe in U.S. Pat. No. 3,976,899, for example. In such a switch, the amplitude of the voltage pulse of the piezoelectric film switch is directly proportional to the magnitude of the applied stress, and hence to the resulting strain. Also, since the applied stress acts upon the film's cross-section, deforming the film by stretching its length will maximize the stress and, therefore the resulting output voltage. These characteristics of the piezoelectric switch render it quite practical for use as a switching element in a keyboard, keypad and the like. Further examples of switches including piezoelectric films are described in U.S. Pat. No. 4,158,117 to Quilliam et al; U.S. Pat. No. 4,328,441 to Kroeger, Jr., et al: U.S. Pat. No. 3,935,485 to Yoshida et al and U.S. Pat. No. 4,137,471 to Yoshida et al. See also copending U.S. patent application Ser. No. 07/429,381, entitled "Dual Direction Switch."
Piezoelectric film switches have distinct advantages over conventional mechanical switches in that they are not susceptible to malfunction by contaminants such as dirt, moisture and abrasive dust. Also, since a piezoelectric film switch operates essentially by developing a charge within the film and transferring that charge to the film's outer electrodes, no mechanical closure or opening is required to make or break an electrical contact. Moreover, unlike membrane keyboards which radiate high frequencies during an electronic scan, piezoelectric film switches operate at low current and generate minimal RF levels. Piezoelectric film switches also do not experience "contact bounce" and therefore greatly simplify circuit design.
Additional information relating to the structure, properties, applications and fabrication of piezoelectric film switches, as well as piezoelectric films in general, can be found in the KYNAR.RTM. Piezo Film Technical Manual (1987), which is available from ATOCHEM. That manual is incorporated herein by reference.
Previous attempts also have been made to incorporate the above-mentioned benefits of a piezoelectric film switch into a snap action type switch. For example, in U.S. Pat. No. 4,383,195, Kolm et al disclose a piezoelectric snap actuator including a piezoelectric element and a snap action device for exerting a force in opposition to the piezoelectric element. The snap action device has a predetermined reaction force that must be overcome by the applied force of the piezoelectric element to cause the snap action. For this purpose, an electric field is applied to the piezoelectric element to enable it to generate an opposing force in excess of the reaction force of the snap action device and to store energy in the snap action device necessary to enable occurrence of the snap action.
However, the device of Kolm et al is ineffective for converting a mechanical action into an input signal because it instead converts an electrical signal into a mechanical action. In other words, the snap action switch of Kolm et al operates by applying voltage pulses to the piezoelectric element until the snap action device changes state. On the other hand, when a snap action switch is used as an input device, a mechanical force is applied to the snap action device until the piezoelectric element generates a signal indicating that the snap action device has been depressed by a user. The present invention is of the latter type.
A piezoelectric switch of the latter type has been disclosed by Ohigashi et al in U.S. Pat. No. 3,940,637. Ohigashi et al describe a switch containing a convex elastic plate made of a copper-beryllium alloy which is mounted on a substrate containing an aperture. A piezoelectric film having electrodes on both sides thereof is adhered to the convex plate above the aperture so that when a force is applied downwardly on the combination of the convex plate and the piezoelectric film the convex disc flexes to create the snap action. The snap 15 action of the convex plate thereby induces strain in the piezoelectric film to produce an output voltage. The switch may be further simplified by forming the piezoelectric film to have a snap action characteristic so that the piezoelectric film itself forms the key switch.
Switches of the type disclosed by Ohigashi et al are particularly useful as key switches because the switch provides a predetermined output for a predetermined strain velocity by prohibiting an output signal from being generated unless sufficient force is applied to the key switch by an operator's finger. In other words, only when the elastic plate of Ohigashi et al is sufficiently strained by an operator's finger will a pulse signal having a predetermined value and form be generated by the piezoelectric film through its layer electrodes. Also, since the elastic plate exerts a snap action, just after the input force or the enforced strain reaches the predetermined value, the strain increases considerably instantaneously. Then, upon removal of the input force, the elastic plate returns to its original state.
However, the piezoelectric switch disclosed by Ohigashi et al has several problems. Namely, because the piezoelectric film of the switch is directly responsive to an externally applied force, the piezoelectric film may be subject to wear, adversely affecting the switch's durability. In addition, it is difficult to attach external electrical conductors to the switch due to the lack of reliability of connection of metal conductors to the piezoelectric film. Moreover, when a vibrational force is detected by the switch of Ohigashi et al, the piezoelectric film is subjected to stress or strain which may inadvertently cause an output signal to be generated. Such vibration induced signals may cause spurious output signals from the keypad and hence spurious input signals to the responsive circuitry.
As a result of the above-mentioned deficiencies in the prior art, there has been a long-felt need to provide a more rigid piezoelectric snap action switch which is not affected by vibration and which may be readily connected to metallic conductors.